Explicit and Implicit Hybrid Beamforming Channel Sounding

ABSTRACT

A channel sounding scheme is presented herein that relies on a combination of a first channel sounding procedure and a second channel sounding procedure. The first channel sounding technique is one that involves an exchange of dedicated channel sounding related signals to determine channel conditions between the first wireless communication device and the particular second wireless communication device. The second channel sounding technique is one in which channel conditions are implicitly discovered from any signals transmitted by the particular second wireless communication device to the first wireless communication device. A first wireless communication device computes updates to steering matrix information used for beamforming one or more signal streams to a particular second wireless communication device based on a combination of the first channel sounding technique and the second channel sounding technique.

TECHNICAL FIELD

The present disclosure relates to wireless communication networks anddevices.

BACKGROUND

In wireless communication systems in which wireless devices havemultiple antennas and use beamforming techniques, the device on at leastone end (or both ends) of a wireless link computes beamforming weightsthat are used when sending a transmission. When one device on the linkis mobile, it is useful to update the beamforming weights to account forchanging conditions in the wireless channel between the two devices.

Channel sounding is a procedure whereby a first device learnsinformation about the wireless channel with respect to a second device,so that the first device can update its beamforming weightsappropriately for use when sending a transmission to the second device.Some wireless communication systems operate in accordance with awireless communication standard, e.g., the IEEE 802.11 standard forwireless local area networks (WLANs). The IEEE 802.11 standard specifieswhat is called an “explicit” beamforming feedback sounding procedure inwhich an access point (AP) and a client device follow a particularexchange of signals to convey information to the AP about the wirelesschannel between the AP and the client device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless communication system in which afirst wireless communication device is configured to perform a hybrid ofa first channel sounding technique and a second channel soundingtechnique with respect to one of a plurality of second wirelesscommunication devices.

FIG. 2 is a flow chart depicting a procedure for limiting overheadassociated with the first channel sounding technique.

FIGS. 3 and 4 are flow charts illustrating examples of handling channelinformation resulting from the first and second channel soundingtechniques, respectively.

FIG. 5 is an example of a flow chart for a procedure to combine channelinformation from the first and second channel sounding techniques.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

A channel sounding scheme is presented that relies on a combination of afirst channel sounding procedure and a second channel soundingprocedure. A first wireless communication device, having a plurality ofantennas and capable of wirelessly communicating with one or more secondwireless communication devices having one or more antennas, computesupdates to steering matrix information used for beamforming one or moresignal streams to a particular second wireless communication devicebased on a combination of the first channel sounding technique and thesecond channel sounding technique. The first channel sounding techniqueis one that involves an exchange of dedicated channel sounding relatedsignals to determine channel conditions between the first wirelesscommunication device and the particular second wireless communicationdevice. The second channel sounding technique is one in which channelconditions are implicitly discovered from any signals transmitted by theparticular second wireless communication device to the first wirelesscommunication device.

Example Embodiments

A hybrid of first and second channel sounding techniques is presentedfor use between first and second wireless communications devices, wherethe first wireless communication device is configured to beamformtransmissions to the second wireless communication device via aplurality of antennas of the first wireless communication device. Thefirst wireless communication device uses a steering matrix to weight oneor more signal streams across its plurality of antennas to be beamformedto the second wireless communication device. The first wirelesscommunication device may be, for example, a wireless access point (AP)that is configured to wirelessly communicate with any one or more secondwireless communication devices, e.g., client devices. Updates to thesteering matrix information used for beamforming one or more signalstreams to a particular second wireless communication device is based ona combination of the first channel sounding technique and the secondchannel sounding technique.

The first channel sounding technique involves an exchange of dedicatedchannel sounding related signals to determine channel conditions betweenthe first wireless communication device and the particular secondwireless communication device. The second channel sounding technique isone in which channel conditions are implicitly discovered from any(uplink) signals transmitted by the particular second wirelesscommunication device to the first wireless communication device.

The following description makes reference to the IEEE 802.11 wirelesscommunication standard, by way of example only. The 802.11nspecification defines several beamforming modes, including an “explicit”mode and an “implicit” mode.

The IEEE 802.11 implicit channel sounding procedure involves an APrequesting a client device to send a specific sounding frame that doesnot carry any data, and this sounding procedure only works when both theAP and client device support the standard-based implicit soundingprocedure. Similarly, the 802.11 explicit sounding procedures requirespecific support on both the AP and client device, and the use ofspecific frames transmitted between the AP and client. An AP can supportnone, one, or all the beamforming channel sounding modes in the 802.11specification. A client device may not support the explicit soundingprocedure of the 802.11 standard.

In the hybrid channel sounding scheme presented herein, the firstchannel sounding technique may be the 802.11 explicit soundingprocedure. The second channel sounding technique is one in which thechannel conditions are determined from any (uplink) signals transmittedby the client device to the AP in the normal course of data transfer,and does not require any specific participation/support from the clientdevice.

It should be understood that these techniques are not limited to an IEEE802.11 WLAN, and may be used in any wireless communication system thatemploys beamforming techniques, such as the Long Term Evolution (LTE)wireless communication standard, etc.

The weaknesses/strengths of each of these sounding techniques complimentone another. Enabling only one method per radio or per client deviceleaves holes in the solution, even for client devices that fully supportthe first channel sounding technique.

For client devices that fully support the first channel soundingtechnique, channel overhead is still required for sounding if only usingthe first channel sounding technique. In certain cases of highaggregation, this overhead may not be noticeable. But in many trafficscenarios with numerous client devices, sounding before every downlinktransmission can be cumbersome. The second channel sounding technique isguaranteed to have full channel sounding for (single antenna, singlespatial stream, i.e., 1×1) client devices upon receiving eachacknowledgement (ACK) frame, so spending any channel resources primarilyfor sounding those client devices is wasteful. Furthermore, for mostsituations, the second channel sounding technique will yield fullchannel sounding even for client devices that receive more than onespatial stream. Spending channel resources primarily to sound thoseclient devices is also wasteful.

From a channel sounding perspective, the first channel soundingtechnique provides a benefit over the second channel sounding techniqueonly when sounding the channel to a multiple spatial stream clientdevice (which is capable of supporting the first channel soundingtechnique) when the uplink signals are not sufficiently sounding thechannel (e.g., when the client data rate shifts to fewer than themaximum number of spatial streams, when there are uplink signals as inpure User Datagram Protocol (UDP) traffic, etc.).

One disadvantage of the first channel sounding technique is that thebeamformee (client device) determines the precoding that the AP will usein beamforming on the downlink. Many client devices will havesub-optimal precoding computation capability. The second channelsounding technique, with the computation performed by the AP (insoftware, for example) has much greater flexibility in approachingoptimal precoding. Thus, the hybrid channel sounding scheme presentedherein uses two different types of channel sounding techniquesconcurrently to leverage their respective advantages.

Reference is now made to FIG. 1. FIG. 1 illustrates an example of ablock diagram of a system 10 comprising a first wireless communicationdevice 20 (e.g., an AP) and a plurality of second wireless communicationdevices (e.g., client devices) 30(1)-30(N).

The first wireless communication device 20 comprises a basebandprocessor (e.g., modem) 22, a controller 24, memory 25, a plurality ofreceivers 26(1)-26(M), a plurality of transmitters 27(1)-27(M) and aplurality of antennas 28(1)-28(M). Each receiver is coupled to acorresponding one of the antennas 28(1)-28(M) and each transmitter iscoupled to a corresponding one of the antennas 28(1)-28(M). That is,receiver 26(1) and transmitter 27(1) are coupled to antenna 28(1),receiver 26(2) and transmitter 27(2) are coupled to antenna 28(2), andso on.

Each receiver 26(1)-26(M) converts a signal detected by its associatedantenna to an antenna-specific (baseband) receive signal that issupplied to the baseband processor 22. The baseband processor 22performs baseband processing on the antenna-specific (baseband) receivesignals. The baseband processor 22 also generates a plurality ofantenna-specific (baseband) transmit signals, each of which is suppliedto a corresponding one of the transmitters 27(1)-27(M). The basebandprocessor 22 may be implemented in hardware by digital logic gates, inthe form of one or more application specific integrated circuits(ASICs). Alternatively, the baseband processor 22 may be implemented asa general purpose processor that executes software.

The controller 24 is, for example, a microprocessor or microcontrollerthat performs high level control functions of the first wirelesscommunication device 20. For example, the controller 24 may execute oneor more software programs stored in the memory 26 for performing itscontrol functions. The controller 24 may also supply the raw data to thebaseband processor 22 to be baseband modulated/formatted fortransmission via the transmitters 27(1)-27(M) and consumes the datacontained in the antenna-specific (baseband) receive signals recoveredby the baseband processor 22.

The memory 25 may comprise read only memory (ROM), random access memory(RAM), magnetic disk storage media devices, optical storage mediadevices, flash memory devices, electrical, optical, or otherphysical/tangible memory storage devices. In general, the memory 25 maycomprise one or more tangible (non-transitory) computer readable storagemedia (e.g., a memory device) encoded with software comprising computerexecutable instructions and when the software is executed (by thecontroller 24) it is operable to perform the operations describedherein. Either the baseband processor 22 or controller 24, or acombination of both, may be configured to perform the hybrid channelsounding techniques presented herein.

The first wireless communication device 20 can communicate with any oneor more of the second wireless communication devices 30(1)-30(M), e.g.,according to the rules of the IEEE 802.11 specification. Each of thesecond wireless communication devices 30(1)-30(M) has one or a pluralityof antennas. For example, wireless communication device 30(1) has twoantennas, wireless communication device 30(2) has one antenna andwireless communication device 30(N) has two antennas.

In terms of sounding the wireless channel with respect to any secondwireless communication device, the first wireless communication device20 uses a hybrid of the aforementioned first channel sounding techniqueor second channel sounding technique. For example, FIG. 1 shows thefirst channel sounding technique at reference numeral 40 and the secondchannel sounding technique at reference numeral 50. In particular, FIG.1, shows that client devices 30(1) and 30(N) are each capable ofperforming the first channel second technique, where client device 30(2)is not. Thus, the first wireless communication device 20 has twosounding procedures available for client devices 30(1) and 30(N), andonly one available for client device 30(2).

As explained above, the first channel sounding technique may be the802.11 explicit sounding procedure and the second channel soundingtechnique is one in which the channel conditions are determined from any(uplink) signals transmitted by a second wireless communication deviceto the first wireless communication device in the normal course of datatransmissions.

As explained in more detail hereinafter, the AP computes updates tosteering matrix information used for beamforming one or more signalstreams to a particular client device based on a combination of a firstchannel sounding technique that involves an exchange of dedicatedchannel sounding related signals to determine channel conditions betweenthe AP and the particular client device and a second channel soundingtechnique in which channel conditions are implicitly discovered from anysignals transmitted by the particular client device to the AP.

FIGS. 2-5 will now be referred to for a more detailed presentation ofthe hybrid channel sounding scheme comprising non-standard implicitmultiple spatial stream beamforming (i.e., the second channel soundingtechnique) with coordinated explicit sounding (i.e., the first channelsounding technique). For client devices that do not support the firstchannel sounding technique (e.g., explicit beamforming feedback), thesecond channel sounding scheme is used exclusively. For all other clientdevices, the following scheme is used.

Limiting Explicit Beamforming Sounding (First Channel SoundingTechnique) Overhead

Referring now to FIG. 2 (with continued reference to FIG. 1), a process100 is described for limiting overhead associated with the first channelsounding technique performed in the first wireless communication device20 (e.g., AP). The operations described with reference to FIG. 2 may beperformed by the baseband processor 22 or controller 24. At 110, unicastor multi-user packets are passed down from the host (e.g., thecontroller 24) to the baseband processor 22. Several operations areperformed to determine if certain conditions are met before initiating asounding exchange according to the first channel sounding technique.First, it is determined whether the destination media access control(MAC) address for the transmission is associated with a client devicethat the AP 10 knows, from prior exchanges with that client device,supports the first channel sounding technique.

It is also determined whether the destination MAC address (orbeamforming slot identifier) has stale beamforming weights, i.e., astale steering matrix. Timestamps are tracked when updates to thesteering matrix are made. This prioritizes available sounding overheadfor client devices that need a steering matrix update. A steering matrixcan be deemed “stale” (done individually for each bandwidth of 20 MHz/40MHz/80 MHz/160 MHz, though an update to a given bandwidth is also anupdate to all sub-bandwidths) if it has been greater than X microsecondssince the last full steering matrix update, where X is the same for allclient devices and is set to a general channel coherence time targetvalue, or X is set individually for each client device and is equal toan estimation of each client device's coherence time.

Thus, at 112, the clock of the AP 20 is compared with a timestamp1corresponding to the time at which the last full sounding was made forthat client device to determine an elapsed time. At 114, this elapsedtime is compared with a first threshold (threshold1), and if this firstthreshold is not exceeded, then there is no need to do a sounding, asindicated at 124 in FIG. 1. On the other hand, if the elapsed time doesnot exceed the first threshold, then the process continues. Thus, uponqueuing of data to be transmitted or at transmit time of the data to theparticular client device, the AP determines whether a first period oftime has elapsed since execution of the first channel sounding techniquefor the particular client device. The determination as to whether thefirst period of time has elapsed is based on a coherence time of awireless channel between the AP and the particular client device.

At 116, the elapsed time since the AP 20 made an explicit beamforming(EBF) sounding exchange (i.e., the first channel sounding technique) toany client device is determined. At 118, this elapsed time is comparedto a second threshold (threshold2). If this second threshold is notexceeded, then there is no need to do a channel sounding and the processends at 124. On the other hand, if the second threshold is exceeded, theprocess continues. Operations 116 and 118 are performed to ensure thatthe sounding overhead is limited to a certain target. For example, theattempted downlink data rate of a packet is at least some data rate Z.This further prioritizes allocation of sounding overhead to clientdevices that need it. A 3×3 (three antenna, three spatial stream) clientdevice that will not support anything close to a three spatial streamdata rate on the downlink will not benefit from a fully sounded channel.

In summary, when it is determined that the first period of time haselapsed since the first channel sounding technique has been performedfor the particular client device, it is further determined whether asecond period of time has elapsed since execution of the first channelsounding technique by the AP for any client device. The first channelsounding technique is executed for the particular client device when itis determined that the second period of time has elapsed since the firstchannel sounding technique has been performed by the AP for any clientdevice.

In a variation, when it is determined that the second period of time haselapsed since the first channel sounding technique has been performed bythe AP for any client device, the AP sends to the particular clientdevice a frame configured to trigger initiation of the first channelsounding technique or second channel sounding technique.

At 120, a timestamp, timestamp2, is updated to the current time of theAP's clock in order to timestamp the time when the first channelsounding technique is invoked. At 122, the first channel soundingtechnique is initiated by the AP, e.g., the AP sends a Null Data PacketAnnouncement (NDPA) according to IEEE 802.11 to initiate an explicitbeamforming feedback channel sounding procedure for that client device.

Turning to FIGS. 3 and 4, handling of channel information from eithersounding technique is now generally described. In FIG. 3, at 140, the AP20 receives explicit beamforming feedback from a client device as partof the first channel sounding technique. At 142, the AP writes theexplicit feedback information to a dedicated explicit feedback buffer toretain the explicit sounding feedback information transmitted by aclient device to the AP resulting from execution of the first channelsounding technique. In FIG. 4, at 150, when the AP receives a packetfrom a client device in the normal course of a data transmission fromthe client device to the AP, the AP computes channel state information(CSI) based on reception of the packet on its multiple antennas as partof the second channel sounding technique. At 152, the AP writes the CSIto a dedicated implicit CSI buffer. Thus, the AP retains CSI that itgenerates resulting from execution of the second channel soundingtechnique.

Mixing First and Second Channel Sounding Techniques

Reference is now made to FIG. 5 for a description of an exampleprocedure by which an AP combines channel information derived from thefirst and second channel sounding techniques. Explicit beamformingfeedback information, generated by a client device from the firstchannel sounding technique, is triaged in batches along with implicitCSI generated by the AP from received uplink transmissions. At 160, itis determined that it is time to triage explicit beamforming feedbackinformation and implicit CSI based on a CSI Triage Interrupt event. At162, the explicit beamforming feedback information and implicit CSI istriaged. In triage, a cost function is applied to implicit CSI andexplicit beamforming feedback information. For an individual clientdevice, the sounding technique that results from the greater number ofsounded signal streams, wider frequency bandwidth and more recenttimestamps is selected to be processed into a steering matrix update.For example, a cost function can be used:

C=a*BW+b*nSS+c*Telapsed+d(client)*isImplicit

where BW is the bandwidth of the channel used between the client deviceand access point, nSS is the number of spatial streams to be transmittedto the client device by the access point, Telapsed is the timestamp onthe implicit CSI or explicit feedback minus the timestamp of the lastfull sounding of the client device (i.e.,Telapsed=timestampLastFullSounding(client)−timestamp(CSIentry orexplicit feedback)), and isImplicit takes on a value depending onwhether or not the entry is implicit CSI. The variables a, b, c and dare weights or factors that can be used to tune system behavior. The“d(client)” weight is used to penalize explicit feedback for clientdevices that are determined to have bad explicit feedback (the clientdevice has been flagged). In other words, the cost function used forselecting use of either explicit sounding feedback information orimplicit channel state information is based on an evaluation of theexplicit sounding feedback information for the particular client device.

Thus, a selection is made for use in computing updates to the steeringmatrix information of either the explicit sounding feedback informationor the implicit CSI as a function of (using a cost function based on)the number of signal streams sounded for the wireless channel, frequencybandwidth sounded for the wireless channel and time proximity (i.e.,recentness).

At 164, if the triage operation 162 selects the implicit CSI, theimplicit CSI is run through a singular value decomposition (SVD)operation, an implicit phase calibration is applied in the SVDcomputation, and the phase calibrated SVD results are saved to/retainedin memory. If the explicit beamforming feedback information is selected,it is decompressed, converted to the proper number of bits andsubcarrier grouping and saved to/retained in memory. No phasecalibration is applied to steering matrix information derived from theexplicit beamforming feedback information. Thus, phase calibration isadaptively applied in computing the update of the steering matrixinformation depending on whether the explicit sounding feedbackinformation or the channel state information is selected for use incomputing the update of the steering matrix information.

For ease of reference hereinafter, the term “implicit steering matrix”refers to the steering matrix derived from the implicit CSI and the term“explicit steering matrix” refers to the steering matrix derived fromthe explicit beamforming feedback information.

Validity Check of Steering Matrix Derived from Explicit BeamformingFeedback Information

At 166, the validity of the explicit beamforming feedback information isevaluated. After decompression of the explicit beamforming feedbackinformation, the explicit beamforming feedback information is checkedfor the property V^(H)V˜=I, where V is the steering matrix, I is theidentity matrix and ^(H) is the Hermitian operation. For example, thecomputation:

10*log 10((sum(sum(|V ^(H) V| ²))−trace(|V ^(H) V|2))/trace(|V ^(H) V|²))<XdB_orth (where X is some threshold value)

may be used to check whether the power in the diagonal elements ofV^(H)V is much greater than the non-diagonal elements. If theorthogonality test does not pass, the client device is flagged. When aclient device is flagged, several actions are possible: (1) increase thefirst threshold (threshold1) to deprioritize that client device'sexplicit feedback information, (2) de-weight the explicit feedbackinformation in the triage operation (at 162), or (3) perform both (1)and (2). Thus, the first threshold, threshold1, may be adjusted based onan evaluation of the explicit sounding feedback information for theparticular client device.

The explicit steering matrix will produce the implicit CSI when receivedat the AP. The implicit CSI so produced represents the AP's channelestimation within the coherence time of the explicit beamformingfeedback sounding. The implicit steering matrix can be compared to theexplicit steering matrix, though the implicit steering matrix willlikely have fewer columns, which is the reason explicit beamformingfeedback sounding was needed.

The computation |V_imp^(H)V_exp|² may be used to show how much eachcolumn of the implicit steering matrix V_imp is projected into thecolumns of the explicit steering matrix V_exp. If the first eigenmodesof the explicit steering matrix and implicit steering matrix are nothighly correlated (>0.95), the steering matrix for that client devicewill be flagged as potentially sub-optimal and the AP may have therestriction that only the steering matrix information resulting from afully sounded channel is updated from explicit beamforming feedbackinformation. For example, for a 3×3 client device, only the 3-spatialstream steering matrix will be updated, and not the steering matrix forthe 2-spatial stream or single spatial stream) since the client deviceappears to be mixing up the eigenmodes.

At 168, the AP computes updates to the steering matrix information forall client devices that the AP is serving. At 170, it is determinedwhether the sounding was a full channel sounding. If a full channelsounding was made, then at 172, after the steering matrix information isupdated for each client device, the timestamp1 for the particular clientdevice is updated based on the timestamp when the first channel soundingtechnique or second channel sounding technique was performed. Operation174 is arrived at either if the determination step 170 resulted in anegative determination or after operation 172. At operation 174, thecoherence time estimate/first threshold is updated for all clientdevices of the AP.

For a given client device, the AP may compute updates to the steeringmatrix information from implicit CSI and from explicit beamformingfeedback information. For example for a 3-spatial stream client device,a 3-spatial stream steering matrix may be computed from explicitbeamforming feedback information while the steering matrix for 2-spatialstreams and a single spatial stream is computed from more recentimplicit CSI. Said another way, within a single user, updates to thesteering matrix information may use a mix of explicit beamformingfeedback information (resulting from execution of the first channelsounding technique) and implicit CSI (resulting from execution of thesecond channel sounding technique) such that over time some steeringmatrix information updates use implicit CSI and other steering matrixinformation updates use explicit beamforming feedback information.

Described above are examples. The concepts described herein may beembodied in other specific forms without departing from the spirit oressential characteristics thereof. The foregoing examples are thereforeto be considered in all respects illustrative and not meant to belimiting. Accordingly, it is intended to embrace all such alterations,modifications and variations that fall within the spirit and scope ofany claims filed in applications claiming priority hereto interpreted inaccordance with the breadth to which they are fairly, legally andequitably entitled.

What is claimed is:
 1. A method comprising: at a first wirelesscommunication device having a plurality of antennas and capable ofwirelessly communicating with one or more second wireless communicationdevices having one or more antennas, computing updates to steeringmatrix information used for beamforming one or more signal streams to aparticular second wireless communication device based on a combinationof a first channel sounding technique that involves an exchange ofdedicated channel sounding related signals to determine channelconditions between the first wireless communication device and theparticular second wireless communication device and a second channelsounding technique in which channel conditions are implicitly discoveredfrom any signals transmitted by the particular second wirelesscommunication device to the first wireless communication device.
 2. Themethod of claim 1, further comprising determining whether a first periodof time has elapsed since execution of the first channel soundingtechnique for the particular second wireless communication device whenit is time to transmit data to the particular second wirelesscommunication device.
 3. The method of claim 2, wherein determiningwhether the first period of time has elapsed is based on a coherencetime of a wireless channel between the first wireless communicationdevice and the particular second wireless communication device.
 4. Themethod of claim 2, when it is determined that the first period of timehas elapsed since the first channel sounding technique has been executedfor the particular second wireless communication device, furthercomprising: determining whether a second period of time has elapsedsince execution of the first channel sounding technique by the firstwireless communication device for any second wireless communicationdevice; and executing the first channel sounding technique for theparticular second wireless communication device when it is determinedthat the second period of time has elapsed since an explicit soundingprocedure has been performed by the first wireless communication devicefor any second wireless communication device.
 5. The method of claim 2,when it is determined that the first period of time has elapsed sinceexecution of the first channel sounding technique for the particularsecond wireless communication device, further comprising: determiningwhether a second period of time has elapsed since the first channelsounding technique has been performed by the first wirelesscommunication device for any second wireless communication device; andsending to the particular second wireless communication device a frameconfigured to trigger initiation of the first channel sounding techniqueor the second channel sounding technique when it is determined that thesecond period of time has elapsed since the first channel soundingtechnique has been performed by the first wireless communication devicefor any second wireless communication device.
 6. The method of claim 2,further comprising: retaining explicit sounding feedback informationtransmitted by the particular second wireless communication device tothe first wireless communication device resulting from execution of thefirst channel sounding technique; and retaining channel stateinformation generated by the first wireless communication deviceresulting from execution of the second channel sounding technique. 7.The method of claim 6, further comprising selecting for use in computingupdates to the steering matrix information either explicit soundingfeedback information or the channel state information as a function ofthe number of signal streams sounded for the wireless channel, frequencybandwidth sounded for the wireless channel and time proximity.
 8. Themethod of claim 7, further comprising adaptively applying phasecalibration in computing the update of the steering matrix informationdepending on whether the explicit sounding feedback information or thechannel state information is selected for use in computing the update ofthe steering matrix information.
 9. The method of claim 7, furthercomprising adjusting a cost function used for the selecting based on anevaluation of the explicit sounding feedback information for theparticular second wireless communication device.
 10. The method of claim6, further comprising adjusting the first period of time based on anevaluation of the explicit sounding feedback information for theparticular second wireless communication device.
 11. The method of claim1, wherein computing comprises computing updates to the steering matrixinformation for the particular second wireless communication device suchthat over time some steering matrix information updates use informationresulting from the first channel sounding technique and other steeringmatrix information updates use information resulting from the secondchannel sounding technique.
 12. An apparatus comprising: a transceiverunit configured to transmit and receive signals via a plurality ofantennas in order to wirelessly communicate with wireless clientdevices; and a processor configured to compute updates to steeringmatrix information used for beamforming one or more signal streams to aparticular wireless client device based on a combination of a firstchannel sounding technique that involves an exchange of dedicatedchannel sounding related signals to determine channel conditions withrespect to the particular wireless client device and a second channelsounding technique in which channel conditions are implicitly discoveredfrom any signals transmitted by and received from the particularwireless client device.
 13. The apparatus of claim 12, wherein theprocessor is further configured to determine a first period of time haselapsed since execution of the first channel sounding technique for theparticular wireless client device when it is time to transmit data tothe particular wireless client device.
 14. The apparatus of claim 13,wherein the processor is further configured to determine whether thefirst period of time has elapsed is based on a coherence time of awireless channel with respect to the particular wireless client device.15. The apparatus of claim 13, wherein when it is determined that thefirst period of time has elapsed since the first channel soundingtechnique has been executed for the particular wireless client device,the processor is further configured to: determine whether a secondperiod of time has elapsed since execution of the first channel soundingtechnique for any wireless client device; and execute the first channelsounding technique for the particular wireless client device when it isdetermined that the second period of time has elapsed since an explicitsounding procedure has been performed for wireless client device. 16.The apparatus of claim 13, wherein when it is determined that the firstperiod of time has elapsed since execution of the first channel soundingtechnique for the particular wireless client device, the processor isfurther configured to: determine whether a second period of time haselapsed since the first channel sounding technique has been performedfor any wireless client device; and send to the particular wirelessclient device a frame configured to trigger the first channel soundingtechnique or the second channel sounding technique when it is determinedthat the second period of time has elapsed since the first channelsounding technique has been performed for any wireless client device.17. The apparatus of claim 16, wherein the processor is furtherconfigured to: retain explicit sounding feedback information transmittedby the particular wireless client device resulting from execution of thefirst channel sounding technique; and retain channel state informationgenerated from execution of the second channel sounding technique. 18.The apparatus of claim 17, wherein the processor is further configuredto select for use in computing updates to the steering matrixinformation either the explicit sounding feedback information or thechannel state information as a function of the number of signal streamssounded for the wireless channel, frequency bandwidth sounded for thewireless channel and time proximity.
 19. The apparatus of claim 18,wherein the processor is further configured to adaptively apply phasecalibration in computing the update of the steering matrix informationdepending on whether the explicit sounding feedback information or thechannel state information is selected for use in computing the update ofthe steering matrix information.
 20. The apparatus of claim 12, whereinthe processor configured to compute updates to the steering matrixinformation for the particular wireless client device such that overtime some steering matrix information updates use information resultingfrom the first channel sounding technique and other steering matrixinformation updates use information resulting from the second channelsounding technique.
 21. One or more computer readable storage mediaencoded with software comprising computer executable instructions andwhen the software is executed operable to: compute updates to steeringmatrix information used for beamforming one or more signal streams froma first wireless communication device to a particular second wirelesscommunication device based on a combination of a first channel soundingtechnique that involves an exchange of dedicated channel soundingrelated signals to determine channel conditions between the firstwireless communication device and the particular second wirelesscommunication device and a second channel sounding technique in whichchannel conditions are implicitly discovered from any signalstransmitted by the particular second wireless communication device tothe first wireless communication device.
 22. The computer readablestorage media of claim 21, further comprising instructions operable to:retain explicit sounding feedback information transmitted by theparticular second wireless communication device to the first wirelesscommunication device resulting from execution of the first channelsounding technique; and retain channel state information generated bythe first wireless communication device resulting from execution of thesecond channel sounding technique.
 23. The computer readable storagemedia of claim 22, further comprising instructions operable to selectfor use in computing updates to the steering matrix information eitherexplicit sounding feedback information or the channel state informationas a function of the number of signal streams sounded for the wirelesschannel, frequency bandwidth sounded for the wireless channel and timeproximity.
 24. The computer readable storage media of claim 22, furthercomprising instructions operable to apply phase calibration in computingthe update of the steering matrix information depending on whether theexplicit sounding feedback information or the channel state informationis selected for use in computing the update of the steering matrixinformation.
 25. The computer readable storage media of claim 21,wherein the instructions operable to compute comprise instructionsoperable to compute updates to the steering matrix information for theparticular second wireless communication device such that over time somesteering matrix information updates use information resulting from thefirst channel sounding technique and other steering matrix informationupdates use information resulting from the second channel soundingtechnique.